As air quality standards become more stringent, considerable efforts have focused on minimizing the particulate matter emitted in diesel engine exhaust. A potential solution is a particulate trap inserted in the exhaust system of a diesel engine.
A honeycomb ceramic wall-flow through filter, such as described in U.S. Pat. No. 4,276,071, has become the preferred type of particulate trap. These honeycomb filters are made by extruding a paste comprised of water, binder and ceramic powders (e.g., clay, mullite, silica and alumina) that form, for example, cordierite upon firing. Clays are generally used to make the paste sufficiently plastic to form useable honeycombs. After the paste is extruded, the honeycomb is dried, debindered and sintered to form a honeycomb. The honeycomb is sintered typically to give sufficient strength to the thin channel walls to survive insertion of a ceramic paste to plug the channels, as described next.
Finally, to make the wall-flow particulate trap or filter, one half of the openings of one end of the sintered honeycomb are plugged with a paste comprised of a suitable powder, dispersion medium and binder. Then on the other end, the channels not already plugged are plugged with the paste. Subsequently, the plugged honeycomb is sintered again to form the wall-flow particulate trap.
Unfortunately, this method suffers from a number of problems. For example, the liquid in the paste may be drawn into the porous walls of the fired honeycomb preferentially causing non-uniform drying shrinkage of the plug and ultimately cracks in the plug. A second problem is the necessity for multiple expensive steps (e.g., at least two high temperature firings) to manufacture the particulate trap. These multiple steps are typically needed because the walls of a green ceramic honeycomb are thin and fragile such that they tend to deform and/or break when inserting the paste. This is particularly true when using a large scale process. Another problem, is the limited compositions that may be used for the plug due to the expansion of the fired honeycomb during the sintering shrinkage of the plug.
Accordingly, it would be desirable to provide a method for making wall-flow traps, for example, that avoids one or more of the problems of the prior art, such as one of those described above.